Electronic device

ABSTRACT

An electronic device for protecting the passengers of a motor vehicle during a collision has an acceleration sensor for generating signals indicative of the deceleration of the vehicle. Switching devices are provided to determine whether the collision is a rear-end collision based on the output signals of the sensor. If a rear-end collision is detected, the electronic device prevents the release of one or more restraining devices, such as an airbag, which would not contribute to the protection of the passengers during such a collision.

FIELD OF THE INVENTION

The present invention relates to electronic devices and, in particular,to electronic devices for controlling the release of passenger restraintdevices, such as airbags, during vehicle collisions.

BACKGROUND INFORMATION

An electronic device for protecting the occupants of a motor vehicleduring a collision is shown in German Patent No. DE 3,621,580 Al.Another such device is shown in U.S. Pat. No. 3,874,695, wherein a motorvehicle is equipped with a plurality of restraining devices which areactivated as a function of the vehicle's speed. For example, arestraining device allotted to a passenger is activated only at slowvehicle speeds, whereas a restraining device allotted to the driver isnot activated at the same speed, or is only activated at a subsequentpoint in time.

German Patent No. 2,151,399 shows how to install several impactsensitive configurations in a vehicle, wherein severalelectro-mechanical sensors have sensitivity axes pointing in differentdirections.

SUMMARY OF THE INVENTION

The present invention is directed to an electronic device forcontrolling the release of a passenger restraint system on a motorvehicle. The electronic device comprises an acceleration sensor mountedon the vehicle for generating output signals indicative of theacceleration of the vehicle. At least one passenger restraint device ismounted on the vehicle and adapted to be activated during a vehiclecollision to protect a passenger in the vehicle. A control unit of thedevice is coupled to the passenger restraint device and to theacceleration sensor. The control unit is adapted to receive the outputsignals from the acceleration sensor and to control the release of thepassenger restraint device in response thereto. The control unit isfurther adapted to determine the direction of the force of a collisionexerted on the vehicle, and to prevent the release of the restraintdevice in response thereto.

In one electronic device of the present invention, the control unitincludes an integrating unit coupled to the acceleration sensor. Theintegrating unit includes a first capacitor for integrating the outputsignals generated by the acceleration sensor. A second capacitor iscoupled in parallel relationship with respect to the first capacitor.First means are coupled to the first capacitor and to the secondcapacitor for monitoring the voltage across the first capacitor. Thefirst means couples the second capacitor to the first capacitor when thevoltage across the first capacitor exceeds a threshold value, thuspreventing the release of the passenger restraint device.

The first means includes a comparator, wherein the inverting inputterminal thereof is coupled to the first capacitor and to the secondcapacitor. The non-inverting input terminal of the comparator is coupledto a voltage divider. The voltage divider is coupled between ground andthe operating voltage of the device, and thus applies a referencevoltage to the non-inverting input terminal of the comparator. A firstswitch is coupled between the output terminal of the comparator and thesecond capacitor. A second switch is coupled between the output terminalof the comparator and the first capacitor. The comparator is adapted tocontrol the positions of the first and second switches to couple thesecond capacitor to the first capacitor when the voltage across thefirst capacitor exceeds a threshold value.

In one electronic device of the present invention, the first meansfurther includes an impedance transformer coupled between the invertinginput of the comparator and the first capacitor. The voltage across thefirst capacitor is applied to the inverting input of the comparatorthrough the impedance transformer.

The present invention is also directed to an electronic device forcontrolling the release of an airbag on a motor vehicle. The electronicdevice comprises an acceleration sensor mounted on the vehicle forgenerating output signals indicative of the acceleration of the vehicle.A control unit is coupled to the airbag and to the acceleration sensor.The control unit is adapted to receive the output signals from theacceleration sensor and to control the release of the airbag in responsethereto.

The control unit includes a first capacitor coupled to the accelerationsensor to receive the output signals generated therefrom. The controlunit further includes a comparator. The inverting input of thecomparator is coupled to the first capacitor. The comparator is adaptedto compare the value of the voltage across the first capacitor to athreshold value. A second capacitor is coupled to the inverting input ofthe comparator and is connected in parallel relationship with respect tothe first capacitor. The control unit further includes means forcoupling the first capacitor to the second capacitor when the voltageacross the first capacitor exceeds the threshold value. The control unitthus prevents the release of the airbag in response to the signalsgenerated by the acceleration sensor.

In one electronic device of the present invention, the means forcoupling includes a first switch having a normally open contact coupledbetween the output of the comparator and the second capacitor. A secondswitch having a normally closed contact is coupled to the output of thecomparator and coupled between the first switch and the first capacitor.The comparator closes the first switch and opens the second switch whenthe voltage across the first capacitor exceeds a threshold value.

The present invention is also directed to a method of controlling therelease of a passenger restraint device on a motor vehicle. The methodof the present invention comprises the following steps: generatingsignals indicative of the acceleration of the vehicle with anacceleration sensor mounted on the vehicle; determining the direction ofthe force of a collision exerted on the vehicle giving rise to theacceleration signals; and based on the direction of the force,controlling the release of the passenger restraint device in responsethereto. Preferably, if the collision exerting the force is a rear-endcollision, the passenger restraint device is not released.

It is not practical to activate an airbag during a rear-end collision,since the passengers typically do not move forward during suchcollisions. Thus, there would likely be no additional protectionafforded by an airbag. One advantage of the device of the presentinvention is that the restraining devices are activated as a function ofthe direction of the actual forces exerted on the vehicle. Therefore,only the particular restraining devices that can contribute to theprotection of the passengers of the vehicle are activated. As a result,the unnecessary activation of restraining devices when they cannotcontribute to the protection of the passengers, or when they mightendanger the passengers, is avoided.

Other advantages of the present invention will become apparent in viewof the following detailed description and drawings taken in connectiontherewith.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an electronic device embodying the presentinvention;

FIG. 2 is a flow chart illustrating conceptually the operational stepsof the device of FIG. 1 in accordance with the present invention;

FIG. 3 is a partial, detailed schematic illustrating the device of FIG.1; and

FIG. 4 includes diagrams a-d showing the signal patterns of the deviceof FIG. 1 generated in response to a rear-end collision.

DETAILED DESCRIPTION

In FIG. 1, an electronic device embodying the present invention forcontrolling the release of passenger restraint devices in a motorvehicle (not shown) is illustrated. The electronic device comprises asensor 10 coupled to a control device 11. The control device 11 is inturn coupled to several restraining devices 12a, 12b and 12c. Therestraining devices are, for example, airbags and/or seat belttighteners, each being allocated to the driver of the vehicle oradditional vehicle passengers. An airbag is typically a protective sackwhich opens in front of the driver or vehicle passengers and is inflatedby pressurized gas during a collision. Airbags thus prevent thepassengers from being injured during a collision.

The sensor 10 preferably includes a piezoelectric accelerometer whichemits a voltage signal indicative of the deceleration of the vehicle.The signal is supplied to the control device 11 which evaluates thesignal. If necessary, the control device 11 activates the restrainingdevices 12a, 12b and/or 12c in response thereto to protect thepassengers in the vehicle. The control device 11 comprises devices fordistinguishing among various types of collisions, as described infurther detail below. For example, the control device 11 can distinguishbetween a head-on collision, a rear-end collision, and a collisionoccurring on a side of the vehicle. Based on the type of collision, itwill activate one or more of the restraining devices 12a, 12b and/or12c.

FIG. 2 is a flow chart illustrating conceptually the operational stepsof the electronic device embodying the present invention. Sequence step100 indicates the start of operation. The device constantly checks ifthe deceleration "a" of the vehicle has reached a limiting value "a0",as indicated by sequence step 101. If the limiting value a0 is reached,the device further checks whether the criterion for a rear-end collisionis present, as indicated by sequence step 102. If a rear-end collisionis at hand, the device ordinarily does not activate the airbagrestraining device, as indicated by sequence step 103. An airbagtypically cannot contribute to the protection of the vehicle passengersduring such a collision. If the collision is not a rear-end collision,the airbag restraining device ordinarily is also activated, as indicatedby sequence step 104.

FIG. 3 is a partial circuit diagram of the electronic device of FIG. 1,illustrating the sensor 10 and control device 11 in further detail. Thesensor 10 is coupled to an integrator 34 which is in turn coupled to adelimiter 35. The output terminal of the delimiter 35 is coupled on oneside to the interconnection point of two current sources 11 and 12. Theother terminal of the current source 12 is coupled to ground and theother terminal of the current source 11 is coupled to the operatingvoltage US. The output terminal of the delimiter 35 is coupled on theother side to a first terminal of an integrating capacitor C31. Thesecond terminal of the integrating capacitor C31 is coupled to theoperating voltage US.

A controller 36, which controls the current sources Il and 12, iscoupled through input terminals to a reference voltage UR and thevoltage UC31 across the integrating capacitor C31. The input terminal ofan impedance transformer 30 is coupled to the first terminal of theintegrating capacitor C31, which turns away from the operating voltagesource US. The output terminal of the impedance transformer 30 iscoupled to the inverting input terminal of a comparator 31.

The non-inverting input terminal of the comparator 31 is coupled to thepick-off of a voltage divider R1, R2. The resistors R1 and R2 arecoupled between ground and the operating voltage US. The first terminalof a resistor R3 is coupled to the pick-off of the voltage divider R1,R2. The second terminal of the resistor R3 is coupled to the outputterminal of the comparator 31, which is in turn coupled to the operatingvoltage US via a resistor R4.

A first switch 32 and second switch 33 are each coupled to andcontrolled by the output terminal of the comparator 31. A first terminalof the first switch 32 is coupled to the input terminal of the impedancetransformer 30 via a capacitor C32. The second terminal of the firstswitch 32 is coupled to the operating voltage US. The first terminal ofthe second switch 33 is coupled to the input terminal of the impedancetransformer 30 via a resistor R5. The second terminal of the secondswitch 33 is coupled to the first terminal of the first switch 32, whichis in turn coupled to the input terminal of the impedance transformer 30via the capacitor C32. As shown in FIG. 3, the first switch 32 has anormally open contact, whereas the second switch 33 has a normallyclosed contact.

The operation of the electronic device of the present invention ishereinafter described with reference to the diagrams of FIG. 4, whereineach pulse shape is plotted as a function of time. The diagrams a-d ofFIG. 4 exhibit specific signal shapes in two time ranges, 1 and 11. Timerange 1 corresponds to the time interval t1-t2, during which adecelerating force acts upon the vehicle, as shown in diagram a.Accordingly, the deceleration, represented by the pulse in diagram a(shown in an idealized shape), acts upon the sensor 10. The decelerationacting upon the sensor 10 is represented in diagram a as a square-wavesignal, beginning at instant t1 and ending at instant t2.

Diagram b shows the output signal generated by the sensor 10 in responseto the deceleration pulse shown in diagram a. The signal corresponds tothe deceleration of the vehicle and thus the physical progression of therear-end collision in the time range I. The shape of the curve indiagram b is a function of the frequency limit of the sensor 10. Thehigh pass time constant of the sensor 10 is indicated as T10.

Diagram c illustrates the output voltage of the integrator 34 generatedin response to the output signal of the sensor 10 shown in diagram b.Curve C1 therefore represents the voltage UC31 across the integratingcapacitor C31. Beginning at a resting level of approximately 3.5 volts,the integration curve C1 climbs in the opposite direction as would bethe case, for example, of the response to a head-on collision requiringa release of an airbag. During time range 11, the output signal of thesensor 10, as shown in diagram b, indicates a deceleration of thevehicle. Therefore, if one were in a position to provide the voltagerange necessary for a smooth integration, it would not result in arelease of the restraining devices 12a, 12b and/or 12c.

In order for the integration process to proceed in the release directionwith accuracy, 50% or more of the available stabilized supply voltage isprovided for this integrating range. As a practical matter, however, theintegration curve C1 of diagram c is not feasible, since five to tentimes the voltage, as compared to the voltage in the case of a head-oncollision, would be required. In the latter case, a voltage within therange of approximately 10 to 20 volts would appear.

In the event of a rear-end collision, the electronic device of thepresent invention typically has an integrating progression which followsthe curve C2 shown in diagram c. However, at most, with the presence ofan internal deceleration threshold, a progression of the integrationcurve according to the dash-dot curve C3 in diagram c is achieved.During the transition from time range I to time range 11, the electronicdevice would release the restraining devices at the instant t3 or t3'.

The voltage UC31 across the integrating capacitor C31, is observed viathe impedance transformer 30, as shown by the circuit diagram of FIG. 3.If this voltage exceeds the threshold S1, as shown in diagram d, thenthe presence of a rear-end collision is determined. The voltagethreshold S1 is set by the resistor combination R1, R2, R3 and R4. Ifthe threshold S1 is exceeded, the first switch 32 is closed by thecomparator 31, and the second switch 33 is opened. As a result, thevoltage at the integrating capacitor C31 is raised to the value US atinstant t1', as indicated in diagram d. The voltage across theintegrating capacitor C31 remains at the value US throughout time rangeI. It then drops slightly at the end of time range I, as indicated bythe dash-dot line D2 in diagram d due to the existing internal currentsource 12 (the deceleration threshold).

Because of the parallel connection of the capacitor C32 and theintegrating capacitor C31, the dis-integration begins in time range 11,as indicated by the curve D1 in diagram d. The integration coefficientis determined by the parallel-coupled capacitors C31 and C32. Athreshold S2 for the voltage UC31 across the integrating capacitor C31is defined by the resistor R3 coupled to the comparator 31. When thevoltage UC31 falls below the threshold S2, the capacitor C32 is disabledby the comparator 31 by opening the first switch 32 and by closing thesecond switch 33.

As will be recognized by those skilled in the art, by changing theintegration coefficients by means of the parallel coupled capacitor C32,the release of the restraining devices can be avoided during a rear-endcollision. The electronic device of the present invention then continuesto operate with the integration time optimized for a head-on collisionafter the capacitor C32 is disabled at instant t4, as shown in diagramd.

With the electronic device of the present invention, there have not beenany observed disadvantageous effects on the release behavior of therestraining devices during a head-on collision. Known electronic deviceswith quartz sensors or piezoelectric sensors for detecting deceleration,on the other hand, have no particular protection against an undesirablerelease during a rear-end collision. Other solutions to this problemhave included the use of a sensor with an extremely low bottom frequencylimit. However, this is technologically very difficult to achieve, aswell requiring a large voltage range for integration. The electronicdevice of the present invention, on the other hand, provides a feasiblesolution to the problem of preventing the release of a passengerrestraint device during a rear-end collision with relatively simpleswitching devices and with substantial accuracy.

As will be further recognized by those skilled in the art, the abilityto protect against the undesirable release of the passenger restraintdevices during a rear-end collision can be optimally adapted to certainvehicle conditions. For example, a rear-end collision can be detectedaccording to mathematical simulations, by simply varying the switchingthresholds S1 and S2, shown in diagram d, as well as by adapting theintegration coefficient by selecting a particularly suitable value forthe capacitor C32.

We claim:
 1. An electronic device for controlling the release of apassenger restraint system on a motor vehicle, comprising:anacceleration sensor mounted on the vehicle for generating output signalsindicative of the acceleration of the vehicle; at least one passengerrestraint device mounted on the vehicle and adapted to be activatedduring a vehicle collision to protect a passenger in the vehicle; acontrol unit coupled to the passenger restraint device and to theacceleration sensor, the control unit being adapted to receive theoutput signals from the acceleration sensor and to control the releaseof the passenger restraint device in response thereto, the control unitbeing further adapted to determine the direction of the force of acollision exerted on the vehicle, and to prevent the release of therestraint device in response thereto, the control unit includinganintegrating unit coupled to the acceleration sensor for generatingintegrated signals based on the output signals generated by theacceleration sensor, and first means for comparing the integratedsignals to a threshold voltage indicative of a rear-end vehiclecollision for preventing the release of the restraint device if anintegrated signal exceeds the threshold voltage.
 2. An electronic deviceas defined in claim 1, whereinthe integrating unit includes a firstcapacitor; and the device further includes a second capacitor coupled inparallel relationship with respect to the first capacitor; and secondmeans coupled to the first capacitor and the second capacitor formonitoring the voltage across the first capacitor and for coupling thesecond capacitor to the first capacitor when the voltage across thefirst capacitor exceeds a threshold value.
 3. An electronic device asdefined in claim 2, wherein the second means includesa comparator, theinverting input terminal thereof being coupled to the first capacitorand to the second capacitor, the non-inverting input terminal thereofbeing coupled to a voltage divider, the voltage divider being coupledbetween ground and the operating voltage of the device, and thusapplying a reference voltage to the non-inverting input terminal of thecomparator; a first switch coupled between the output terminal of thecomparator and the second capacitor; a second switch coupled between theoutput terminal of the comparator and the first capacitor, thecomparator being adapted to control the positions of the first andsecond switches to couple the second capacitor to the first capacitorwhen the voltage across the first capacitor exceeds a threshold value.4. An electronic device as defined in claim 3, wherein the second meansfurther includesan impedance transformer coupled between the invertinginput of the comparator and the first capacitor, the voltage across thefirst capacitor being applied to the inverting input of the comparatorthrough the impedance transformer.
 5. An electronic device forcontrolling the release of a passenger restraint system on a motorvehicle, comprising:an acceleration sensor mounted on the vehicle forgenerating output signals indicative of the acceleration of the vehicle;at least one passenger restrain device mounted on the vehicle foractivation during a vehicle collision to protect a passenger in thevehicle; a control unit coupled to the passenger restraint device and tothe acceleration sensor for receiving the output signals from theacceleration sensor and controlling the release of the passengerrestraint device in response thereto, and determining the direction ofthe force of a collision exerted on the vehicle, the control unitincludingan integrating unit coupled to the acceleration sensor, theintegrating unit including a first capacitor for integrating the outputsignals generated by the acceleration sensor; a second capacitor coupledin parallel with the first capacitor; and first means coupled to thefirst capacitor and the second capacitor for monitoring the voltageacross the first capacitor and for coupling the second capacitor to thefirst capacitor when the voltage across the first capacitor exceeds athreshold value to prevent the release of the passenger restraint devicebased on the direction of the force of a collision.
 6. An electronicdevice as defined in claim 5, wherein the first means includesacomparator including an inverting input terminal coupled to the firstcapacitor and to the second capacitor, and a non-inverting inputterminal coupled to a voltage divider, the voltage divider being coupledbetween ground and the operating voltage of the device, and thusapplying a reference voltage to the non-inverting input terminal of thecomparator; a first switch coupled between the output terminal of thecomparator and the second capacitor; and a second switch coupled betweenthe output terminal of the comparator and the first capacitor, whereinthe comparator controls the positions of the first and second switchesto couple the second capacitor to the first capacitor when the voltageacross the first capacitor exceeds a threshold value.
 7. An electronicdevice as defined in claim 6, wherein the first means further includesanimpedance transformer coupled between the inverting input of thecomparator and the first capacitor, the voltage across the firstcapacitor being applied to the inverting input of the comparator throughthe impedance transformer.
 8. A method of controlling the release of apassenger restraint device on a motor vehicle, comprising the followingsteps:generating signals indicative of the acceleration of the vehiclewith an acceleration sensor mounted on the vehicle; determining thedirection of the force of a collision exerted on the vehicle giving riseto the acceleration signals for controlling the release of the passengerrestraint device based on the direction of the force; integrating thesignals generated by the acceleration sensor with an integratingcapacitor; applying the voltage across the integrating capacitor to theinverting input terminal of a comparator; applying a reference voltageto the non-inverting input terminal of the comparator; comparing thevoltage across the integrating capacitor to a threshold voltage; andwhen the voltage across the integrating capacitor exceeds the thresholdvoltage, coupling another capacitor in parallel with the integratingcapacitor to prevent the release of the passenger restraint device basedon the direction of the force.
 9. An electronic device for controllingthe release of a passenger restraint system on a motor vehicle,comprising:an acceleration sensor mounted on the vehicle for generatingoutput signals indicative of the acceleration of the vehicle; at leastone passenger restraint device mounted on the vehicle for activationduring a vehicle collision to protect a passenger in the vehicle; acontrol unit coupled to the passenger restraint device and to theacceleration sensor for controlling the release of the passengerrestraint device in response to the output signals transmitted by theacceleration sensor, the control unit includingan integrating unitcoupled to the acceleration sensor, the integrating unit including afirst capacitor for integrating the output signals transmitted by theacceleration sensor, and a second capacitor for coupling to the firstcapacitor to prevent the release of the passenger restraint device basedon the direction of the force of a collision.
 10. An electronic deviceas defined in claim 9, whereinthe second capacitor is coupled inparallel with the first capacitor; and the control unit further includesmeans coupled to the first capacitor and the second capacitor formonitoring the voltage across the first capacitor and for coupling thesecond capacitor to the first capacitor when the voltage across thefirst capacitor exceeds a threshold value.
 11. A method of controllingthe release of a passenger restraint device on a motor vehicle,comprising the following steps:generating signals indicative of theacceleration of the vehicle with an acceleration sensor mounted on thevehicle; determining the direction of the force of a collision exertedon the vehicle giving rise to the acceleration signals; integrating thesignals generated by the acceleration sensor with an integratingcapacitor; and coupling a second capacitor to the integrating capacitorto prevent the release of the passenger restraint device based on thedirection of the force.
 12. A method as defined in claim 11, wherein ifthe collision exerting the force is a rear-end collision, the passengerrestraint device is not released.
 13. A method as defined in claim 11,further comprising the following steps:applying the voltage across theintegrating capacitor to the inverting input terminal of a comparator;applying a reference voltage to the non-inverting input terminal of thecomparator; comparing the voltage across the integrating capacitor to athreshold voltage; and when the voltage across the integrating capacitorexceeds the threshold voltage, coupling the second capacitor in parallelwith the integrating capacitor.
 14. An electronic device for controllingthe activation of a passenger restraint device on a motor vehicle,comprising:an acceleration sensor mounted on the vehicle for generatingoutput signals indicative of the acceleration of the vehicle; apassenger restrains device mounted on the vehicle and adapted to beactivated during a collision to protect a passenger in the vehicle; acontrol unit coupled to the passenger restraint device and to theacceleration sensor, the control unit being adapted to receive theoutput signals from the acceleration sensor and to control theactivation of the passenger restraint device in response thereto, thecontrol unit includingan integrator coupled to the acceleration sensorfor receiving the output signals therefrom and for generating outputsignals in response thereto, a first capacitor coupled to the integratorfor integrating the output signals generated therefrom, a secondcapacitor coupled in parallel relationship with respect to the firstcapacitor, a comparator, the inverting input terminal thereof beingcoupled to the first capacitor and to the second capacitor, a voltagedivider coupled to the non-inverting input terminal of the comparatorand supplying a reference voltage thereto, a first switch coupledbetween the output terminal of the comparator and the second capacitor,a second switch coupled to the output terminal of the comparator andcoupled between the first switch and the first capacitor,the comparatorbeing adapted to compare the value of the voltage across the firstcapacitor to a threshold voltage value, and when the voltage across thefirst capacitor exceeds the threshold value, to close the first switchand open the second switch and thus couple the second capacitor to thefirst capacitor, thus preventing the activation of the passengerrestraint device.
 15. An electronic device as defined in claim 14,wherein the control unit further comprises:an impedance transformercoupled between the inverting input terminal of the comparator and thefirst capacitor, the voltage across the capacitor being applied to thecomparator through the impedance transformer.
 16. An electronic devicefor controlling the release of an airbag on a motor vehicle,comprising:an acceleration sensor mounted on the vehicle for generatingoutput signals indicative of the acceleration of the vehicle; a controlunit coupled to the airbag and to the acceleration sensor, the controlunit being adapted to receive the output signals from the accelerationsensor and to control the release of the airbag in response thereto, thecontrol unit includinga first capacitor coupled to the accelerationsensor to receive the output signals generated therefrom, a comparator,the inverting input thereof being coupled to the first capacitor, thecomparator being adapted to compare the value of the voltage across thefirst capacitor to a threshold value, a second capacitor coupled to theinverting input of the comparator and connected in a parallelrelationship with respect to the first capacitor, and means for couplingthe first capacitor to the second capacitor when the voltage across thefirst capacitor exceeds the threshold value, to prevent the release ofthe airbag in response to the signals generated by the accelerationsensor.
 17. An electronic device as defined in claim 16, wherein themeans for coupling includesa first switch having a normally open contactcoupled between the output of the comparator and the second capacitor,and a second switch having a normally closed contact coupled to theoutput of the comparator and coupled between the first switch and thefirst capacitor, wherein the comparator closes the first switch andopens the second switch when the voltage across the first capacitorexceeds the threshold value.
 18. An electronic device as defined inclaim 17, wherein the control unit further includesa voltage dividercoupled to the non-inverting input of the comparator and applying areference voltage thereto; and an impedance transformer coupled betweenthe first capacitor and the inverting input of the comparator, thevoltage across the first capacitor being applied to the comparatorthrough the impedance transformer.
 19. A method of controlling therelease of a passenger restraint device on a motor vehicle, comprisingthe following steps:generating signals indicative of the acceleration ofthe vehicle with an acceleration sensor mounted on the vehicle;determining the direction of the force of a collision exerted on thevehicle giving rise to the acceleration signals; integrating the signalsgenerated by the acceleration sensor with an integrating capacitor; andcomparing the voltage across the integrating capacitor to a thresholdvalue indicative of a rear-end vehicle collision, and prevent therelease of the restraint device if the voltage across the integratingcapacitor exceeds the threshold value.
 20. A method as defined in claim19, further comprising the following steps:applying the voltage acrossthe integrating capacitor to the inverting input terminal of acomparator; applying a threshold voltage to the non-inverting inputterminal of the comparator; and when the voltage across the integratingcapacitor exceeds the threshold voltage, coupling another capacitor inparallel relationship with the integrating capacitor.